1. Field of the Invention
The present invention relates to a superconducting high-frequency accelerating tube formed of Nb or the like and used in a charged particle accelerator for accelerating charged particles, and a method for manufacturing the same.
2. Description of the Related Art
Accelerators are based on the principle that charged particles can be accelerated by adjusting the phase velocity of a progressive wave, which has an electric field component in the traveling direction of the charged particles, so that the charged particles are always on the phase of acceleration and are affected by an accelerating electric field. Among these accelerators, ones which utilize a high-frequency electric field use an accelerating tube as means for generating the high-frequency electric field.
A superconducting accelerating tube, made of a superconductor, is used to form a higher-intensity accelerating electric field by means of lower electric power, and niobium (Nb) or the like is used as the material. Since a current excited by means of the high-frequency electric field flows in the vicinity of the inner surface of the accelerating tube, moreover, it is essential to improve the electrical properties of the accelerating tube so that charged particles can flow smoothly, without running out of the tube in the region near the peripheral end portions of inner surfaces at which components of the tube are joined together. Since the superconductivity of the superconductor, such as Nb, is lowered by oxidation, further more, the superconductor should be worked without undergoing oxidation.
In a conventional method for manufacturing superconducting accelerating tubes, a plurality of kinds of components are manufactured by cutting a press-molding a superconducting material, e.g., Nb, and these components are butted to one another at their respective peripheral end portions. Then, the butting portions are welded together to form the accelerating tube having a cell or cells. Among these accelerating tubes, ones which have one cell are called single-cell accelerating tubes, while ones which have a plurality of cells are called multi-cell accelerating tubes.
The TIG (tungsten inert gas) welding process, electron beam welding process, etc. are used to weld those accelerating tubes. Presently, the electron beam welding process is prevailing over other processes, since it facilitates manufacture of accelerating tubes with satisfactory electrical properties.
Using the electron beam welding process for welding the components, however, the conventional method for manufacturing accelerating tubes is subject to the following drawbacks.
In the electron beam welding operation, a plurality of components are butted to one another in a vacuum vessel when they are welded together. In order to cause an electron beam to make a straight advance without scattering, and to prevent Nb or other superconductor from being oxidized, the welding must be executed in a vacuum of 10.sup.-6 torr or less.
Prior to the welding operation, moreover, the components constituting each accelerating tube are assembled into the shape of the accelerating tube in the vacuum vessel, and both ends of the accelerating tube are held by jigs, in order to maintain the shape of the tube, so that the accelerating tube is closed. In welding the components, therefore, the degree of vacuum must be kept uniform in- and outside the accelerating tube, in the vacuum vessel. Conventionally, to cope with this, the jig means is provided with a passage hole or the like, whereby the conductance in- and outside the accelerating tube is increased.
Thus, evacuating the vacuum vessel is so time-consuming that preliminary processes for the welding operation take a lot of time, or equipment, including the vacuum vessel and jigs, is large-scaled, and the jigs is expensive. In consequence, the method using the electron beam welding process entails increased manufacturing cost of the superconducting accelerating tube, and requires highly complicated welding operation.